1. Field of the Invention
The present invention relates to continuously variable transmissions for transmitting drive from a prime mover such as an internal combustion engine to driven members such as the wheels of a vehicle, and more particularly, relates to means for controlling ratio change in an expansible pulley stepless speed changer.
2. Description of the Prior Art
It is known to provide a continuously variable transmission having primary and secondary variable width V-pulleys having a flexible belt or chain of trapezoidal cross section connecting the pair of pulleys and riding in the V-grooves thereof.
Each pulley includes a pair of frusto-conical disc sheaves facing each other defining a V-groove, one of said disc sheaves being axially fixed with respect to a mounting shaft while the other disc sheave is axially movable for varying the width of the V-groove defined between the discs. The radius of engagement of the flexible belt or chain transmission member with a pulley depends upon the axial position of one disc sheave with respect to the other. The axial position of the movable disc sheave in each pulley is adjustable in order to vary the transmission speed ratio between the pulleys. Fluid expansion chamber devices are connected to the movable disc sheaves such that ratio changes are achieved by regulating fluid flow into and from the expansion chambers. A fluid control circuit is connected to primary and secondary expansion chamber devices for regulating the volume of fluid therein and as a result regulates the width of the V-grooves in the pulleys which in turn determines the speed ratio.
A flexible endless transmission band such as a belt or chain of trapezoidal cross section rides in the V-grooves defined in the pair of pulleys. Where the V-groove is narrow, the transmission band rides high in the groove at larger radius, and where the V-groove is wide, the transmission band rides low in the groove at smaller radius.
In a "LOW" speed ratio condition, the primary pulley has a wide V-groove, short radius of contact with the band, and small volume of fluid in the primary expansion chamber, while at the same time, the secondary pulley has a narrow V-groove, large radius of contact with the band, and large fluid volume in the secondary expansion chamber. The opposite condition occurs for "HIGH" speed ratio.
The speed ratio of the pulleys can be varied from "LOW" to "HIGH" by supplying additional fluid to the primary expansion chamber to increase its volume, while at the same time allowing fluid to exhaust from the secondary expansion chamber to reduce its volume. A down shift from "HIGH" to "LOW" occurs where fluid is exhausted from the primary expansion chamber while additional fluid is supplied to the secondary expansion chamber. During ratio changes, the disc sheaves of a pulley must maintain frictional engagement with the opposite edges of the trapezoidal transmission band in order to avoid slippage. In order to maintain appropriate pinch force on the edges of the trapezoidal transmission band while varying the width of the V-grooves in the pulleys, fluid regulating valves are employed for coordinating the admission and release of fluid into and from the primary and secondary expansion chambers.
In a continuously variable transmission according to the prior art, the primary expansion chamber device is provided with a larger effective piston area than the corresponding effective piston area of the seconddary expansion chamber device. The fluid control circuit includes a torque control pressure regulator valve which controls supply and release of fluid from the secondary expansion chamber device at a pressure selected for applying proper pinch force to the opposite edges of the transmission member depending upon the torque to be transmitted to the driven member and the arc of contact between the flexible transmission bond and the sheaves of the secondary pulley. The regulated fluid pressure supplied to the secondary pulley is supplied to a ratio control pressure regulator valve which regulates supply and release of fluid from the primary expansion chamber device at a pressure appropriate for providing non-slipping pinch engagement between the edges of the flexible belt or chain and the conical sheaves of the primary pulley.
In a continuously variable transmission of the type described above, every intermediate speed ratio must be traversed when changing from one speed ratio to another. This is basically a slow process. On the other hand, in a conventional automatic transmission, a ratio change is accomplished by releasing one clutch or brake and engaging a different clutch or brake. This can be done rapidly.
The down shift can be the most difficult ratio change to achieve in a continuously variable transmission inasmuch as, at times, the down shift must be very fast while at the same time proper belt or chain tension must be maintained. During up shift, the ratio will not change without a proper belt tension condition because the secondary pulley system maintains the tension while the primary pulley system must overcome the forces involved in the secondary pulley. In down shifting, the pressure in the primary pulley system could be dropped too rapidly to such a level that the belt could slip and result in damage to the system.
Thus, it is desirable to provide further improvements in controls for continuously variable transmissions permitting rapid down shift while maintaining appropriate tension on the flexible transmission member.